In recent years waterborne paints which employ water as a diluent have become the focus of attention from the viewpoint of air pollution prevention and saving resources and, for example, a mixture of a carboxyl group-containing polyester resin and an aminoplast resin is used in bake-curing type waterborne paints. In the field of mid-coat paints for motor vehicles, in order to confer a sense of being high-class, a high quality of appearance which is attractive to the eye is required, along with which chipping resistance is demanded for protecting the paint material from the impact of small stones and the like, and recently such quality requirements have been steadily increasing so that paint films of excellent painting performance and free of sagging, and which possess excellent smoothness have come to be demanded.
As an example of a novel waterborne mid-coat paint which has a painting performance (in terms of bubble and sag resistance properties) as good as or superior to organic solvent type mid-coat paints, and which is valuable in providing finished paint films of excellent smoothness and top-coat glossiness, there is known a waterborne mid-coat paint in which the chief components are (A) a polyester resin of acid value 10-100, hydroxyl group value 20-300 and number average molecular weight 800 to 10,000, (B) an aqueous amino resin, (C) a linear low molecular weight polyester diol comprising cyclohexanedimethanol and an aliphatic saturated dibasic acid, which has a number average molecular weight of 300-800 and which contains a primary hydroxyl group at both chain ends, and (D) a benzoin alkyl ether (see, for example, JP 2883960). However, this waterborne mid-coat paint has the disadvantage that sagging tends to arise when subjected to pre-heating prior to curing.
Moreover, as a method for obtaining a mid-coat paint film which is outstanding in its hiding properties, chipping resistance and water resistance, and which is free of paint film defects such as sagging and bubbling, there is known a method for forming a paint film using a waterborne paint which includes a hydroxyl group-containing resin, a curing agent comprising a blocked polyisocyanate compound, and/or a melamine resin, and flake-shaped pigment, where the substrate is coated to a coating thickness of 20-35 μm under conditions comprising a solids content at the time of application of 45-70 wt %, a paint viscosity adjusted such that the viscosity at 20° C. is 50-90 seconds measured with a Ford Cup #4, a painting booth temperature of 15-40° C., and a painting booth humidity of 60-90%, and where the viscosity (η) 1 minute following coating is 2000-6000 mPa·sec (20° C., shear rate 2.0/sec) (see, for example, JP-A-2005-034807). However, with this paint film forming method there is the disadvantage that, where the painting booth temperature/humidity comprises a high temperature and low humidity, the coated film viscosity is too high and good smoothness is not obtained.
Again, as a waterborne mid-coat motor vehicle paint having outstanding chipping resistance, which is free from paint film sagging, and gives an excellent finish, there is known a waterborne mid-coat paint for motor vehicles containing (A) a urethane resin aqueous dispersion of resin acid value 2-20 mg/KOH/g, (B) a hydroxyl group-containing resin, and (C) a crosslinking agent, where said urethane resin aqueous dispersion (A) is formed by reacting a polyisocyanate and a polyol having at least two hydroxyl groups per molecule comprising a polyetherpolyol, a polyesterpolyol, a compound having at least one carboxyl group and at least two hydroxyl groups, and, optionally, another polyol, to form a urethane prepolymer, which is neutralized with a neutralizing agent, and then stably dispersed in an aqueous medium (see, for example, JP-A-2006-265310). However, urethane resins are expensive when compared to other resins such as polyesters, and a cheaper method for obtaining a paint material which is outstanding in its appearance and painting performance is desired.
Furthermore, a waterborne mid-coat paint composition is known which includes (A) a water-soluble or water-dispersible polyester resin which is a polyester resin containing, in the resin, 2-10 mass % of linear or branched polyethylene glycol groups or polyethylene glycol ether groups with a number average molecular weight of 300-600, and containing carboxylic acid secondary or tertiary amine salt groups obtained by neutralizing at least some of the carboxylic acid groups in the resin with a secondary or tertiary amine, and which has a combined total of 0.8 to 1.2 of said polyethylene glycol groups or polyethylene glycol ether groups plus said carboxylic acid secondary or tertiary amine salt groups per number average molecular weight of the resin, and the acid value of the polyester resin prior to neutralization of the carboxylic acid groups with the said secondary or tertiary amine is no more than 30 mgKOH/g, the hydroxyl value is 90-160 mgKOH/g and the number average molecular weight is 1,300-1,900, and (B) a melamine resin as the curing agent, and the ratio by mass of the resin solids content of component (A) to that of component (B) is between 75/25 and 90/10 (see, for example, JP-A-2010-248326). However, with this waterborne mid-coat paint composition, the sagging resistance is inadequate when the painting booth temperature/humidity comprises a high temperature and low humidity, and so a method for obtaining a constant painting quality under still broader temperature and humidity conditions is desired.
The present invention provides a waterborne mid-coat paint composition which exhibits very little change in painting performance (in particular, sagging resistance) and appearance along with change in coating conditions like the temperature and humidity in the painting booth, and by means of which a constant painting quality can be obtained over wide-ranging coating conditions.